This application is a U.S. National Stage of International application PCT/FR00/02983, filed Oct. 26, 2000 and published on May 3, 2001 in the French Language.
The present invention relates to contactless transceiver systems, and relates particularly to a high inductance coupling antenna especially used in contactless smart cards.
At present, contactless transceiver systems are widely used in numerous applications. One of these applications is the contactless smart card system which is being increasingly used in various sectors. In the transport sector, such cards have been developed by motorway operators in order to provide users with subscription possibilities and to simplify toll booth payment operations. They have also been developed as a means of payment. This is the case of the electronic wallet, for example. Many companies have also developed identification means for their personnel using contactless smart cards.
The exchange of information between the contactless card and the associated reader is accomplished by remote electromagnetic coupling between an antenna lodged in the contactless card and a second antenna located in the reader. For developing, storing and processing the information, the card is equipped with a chip acting as a memory zone and a microprocessor, which is connected to the antenna. This chip contains an input capacitance provided by capacitors built into the chip. The antenna and the chip are generally located on a flat neutral support. The optimal operation of the antenna-chip coupling, which must not be resistive, is obtained when the following circuit resonance law is observed:
LCxcfx892=1xe2x80x83xe2x80x83(1)
in which L represents the inductance of the antenna, C represents the input capacitance and xcfx89 the pulsation equal to 2xcfx80f, in which f represents the normalized frequency (for example, 13.56 MHz).
The obligation to observe this law requires chip manufacturers, also called founders, to integrate capacitors in the chips in order to obtain sufficiently high capacitance values. In this manner, the production cost of the chips is necessarily higher due to the presence of the capacitors.
The development of contactless smart cards inevitably includes reducing the production cost of the chips used in these cards. In order to reduce the cost of the chips, founders have been increasingly led to reduce the number of capacitors built into the chips and to thereby reduce the capacitance of the circuit. In this manner, manufacturers can produce smaller chips.
In order to observe the law LCxcfx892=1 and to obtain optimal coupling, the inductance L of the antenna must be increased in order to compensate the decrease in the chip""s input capacitance value. In the case of antennas made by using copper aluminum etching techniques, in the form of turns on a plastic dielectric support, the inductance is generally increased by augmenting the number of turns. This solution, however, causes several major drawbacks. Indeed, since any electric circuit has a certain resistance, the increase in the number of turns, which essentially corresponds to an increase in the circuit""s length, leads to a significant increase in the value of this resistance. This considerably affects the performance characteristics of the antenna and thus also the card. As a result, the reading distance of the card is shortened significantly.
In order to limit the overall dimensions and to maintain the effective area for the electromagnetic flow through the card, the width of the copper tracks must be reduced. As a result, the resistance of the antenna is increased and, above all, the reliability of the cards is downgraded as there is a higher risk of the antenna turns breaking when the card bodies are subjected to the hot lamination operation under pressure.
The unit cost of the engraved antenna increases considerably. Thus, the cost reductions obtained by the founders with chips having a lower input capacity are cancelled out by the supplementary cost of the antennas. Card fabrication and use is thus not more profitable.
The object of the invention is to offset these disadvantages by providing an antenna with a high inductance for a high performance card, featuring proven reliability and having a production cost, and thus a cost price, which is much lower than that of the smart cards currently available on the market.
The invention relates to a coupling antenna consisting of a plurality of turns in series located on a flat support made up of an insulating dielectric substrate. This antenna includes one or more assemblies of at least one turn located on said flat support, mounted in series, at least one of the assemblies consisting of at least two turns in series, superimposed in relation to an axis perpendicular to the plane of the support and separated by an insulating strip of dielectric ink enabling a high inductance value to be obtained.
In a preferred embodiment of the invention, the coupling antenna includes one or more assemblies of at least one turn of ink screen-printed on the flat support, mounted in series, at least one of the assemblies being made up of least two turns of screen-printed ink, in series, superimposed along an axis perpendicular to an insulating strip of dielectric ink also screen printed on the support.
Another aspect of the invention is the coupling antenna manufacturing process which consists of:
performing the screen printing of a turn of one or several assemblies by depositing conductive ink on one side of a flat support made up of an insulating dielectric substrate,
performing the screen printing of an insulating strip superimposed on the screen printing of the turn of at least one assembly, by means of a dielectric ink depositing operation, enabling the turn to be covered and to leave the antenna""s bonding pads visible and the connecting zones of the turns superimposed,
performing the screen printing of a turn of at least one assembly, superimposed on the screen printing of the insulating strip by depositing conductive ink,
the second and third steps of the process being repeated one or more times when the antenna features one or more assemblies of more than two superimposed turns.
This antenna and its manufacturing process present many advantages:
a) In order to compensate the higher intrinsic electrical resistivity of screen-printable conductive polymer inks, the cross-section of the antenna""s turns must be increased. This is achieved by widening the turns and/or making a thick deposit of ink. On the basis of these design adaptations, the instantaneous performance characteristics of the screen-printed antenna having less than three turns are at least comparable to those of an engraved antenna and even better after the various mechanical and ageing tests (damp heat). When the inductance of the antenna must be increased in order to match a chip having low internal capacitance, increasing the number of turns is detrimental to the screen-printed antenna as the electrical properties are quickly downgraded above three turns (loss of electrical conductivity and leveling off of the inductance). The inventive process allows this technological dead end to be overcome by offering a screen-printed antenna which is compatible with low-capacitance chips.
b) By modifying the geometric parameters of the coupling antenna according to the invention (thickness of the dielectric insulating layer, width and thickness of the turns, overlapping surface area between superimposed turns), it is possible to adjust the inductance value of the screen-printed antenna in order to obtain perfect matching. An antenna configuration can thus be made which allows the founder to significantly reduce the chip""s input capacitance. This xe2x80x9cexternalizationxe2x80x9d of the capacitance offers very interesting cost reduction prospects for founders.
c) The cost of a screen-printed antenna is practically ten times less than that of an engraved antenna. The implementation of the screen printing of an antenna takes place in accordance with a standard procedure in relation to an antenna screen-printed in the plane (three films, three screens, the same inks). The overall cost of the card is thus much lower as the internal capacitance of the chip has dropped significantly.